This invention pertains to the aircraft control art and, more particularly, to an aircraft optimum lift control system.
Investigations have shown that several aircraft accidents have been caused by severe windshear conditions encountered during takeoff and landing operations. Analysis has shown that the aircraft has sufficient performance capability to avoid most, if not all, of these adverse windshear conditions. The problem has been, basically, one of the flight crew not utilizing the aircraft's performance capabilities in such a manner that the problem can be avoided.
In an attempt to aid the flight crew in utilizing the aircraft's capabilities during climb-out operations, particularly in adverse environmental conditions such as windshear, the prior art has developed numerous climb-out guidance systems. One such system is shown in FIG. 1. Here, an altitude rate sensor 10 produces an output signal h which is representative of aircraft altitude rate. The h signal is compared with an altitude rate bias signal h.sub.B in a summer 12. The altitude rate bias signal h.sub.B is supplied by circuitry indicated by block 14. Block 14 is programed to produce an altitude rate bias signal h.sub.B which establishes a desired rate of aircraft climb. Thus the output from summer 12 is an error signal h.sub..epsilon. equal to the difference between the aircraft's actual altitude rate and the desired rate. The signal h.sub..epsilon. is passed through a shaping filter 16 which, as is well known in this art, enhances system stability. The signal is then summed with a minor loop damping signal, provided by circuitry 18, in a summer 20. As is well known in the aircraft art, minor loop damping signals are generally stabilizing pitch rate signals which prevent short period aircraft oscillation. The resultant output is an elevator command signal .sigma..sub.EC which, when applied to the aircraft's dynamics, controls the aircraft to the commanded climb rate.
FIG. 2 illustrates a second prior art approach which uses a signal proportional to flap position to control the attitude of the aircraft in such a way that its angle of attack follows a pre-programed function. Here, an aircraft angle of attack sensor 30 produces an output signal .alpha..sub.v corresponding to the aircraft's actual angle of attack. This signal is filtered in a filter 32 which performs a similar function to that described with respect to filter 16 of FIG. 1 thereby producing an output signal .alpha..sub.c. This signal is fed as one input to a summer 34.
The sensor 36 produces an output signal .delta..sub.F corresponding to the displacement of the aircraft's flaps. The .delta..sub.F signal is passed to a computer 38 which is programed to output a predetermined angle of attack demand signal .alpha..sub.d as a function of flap position. The summer 34 produces an output error signal .alpha..sub..epsilon. equal to the difference between the aircraft's actual angle of attack .alpha..sub.c and the demanded angle of attack .alpha..sub.d. This signal is summed with path damping signals provided by block 40 in a summer 42. The path damping signals, which are well known in the art, stabilize the aircraft against phugoid perturbations. The output of summer 42 is filtered in filter 44 which provides the same function as filter 16 of FIG. 1, in addition to limiting the maximum value of the signal, and is applied as one input to a summer 46. The other inputs to summer 46 are an output from a minor loop damping block 48, which operates in a manner identical to 18 of FIG. 1, and the output from a pitch attitude block 50. Pitch attitude block 50 produces an output signal which is proportional to the aircraft's pitch attitude. The resultant output from summer 46 is an elevator command signal .delta..sub.EC which is then coupled to the aircraft's dynamics, through the elevator servo control system.
While the second prior art approach does offer the flight crew assistance in climbing out of windshear conditions, both prior systems exhibit numerous shortcomings. For example, the control laws produced by either of the prior art systems are inherently inflexible with respect to variations in aircraft weight and available thrust. In addition, the systems do not totally take into account environmental disturbances, such as windshear and downdraft. In addition, the prior art system of FIG. 2 is critically dependent on pilot action since the basic control parameter (i.e., angle of attack) is a unique function of the position of the flaps. Also, this system is susceptible to errors resulting from the failure of flap position sensors.
Thus, none of the prior art systems takes into account all of the various factors which must be considered in producing an optimum lift control system.